The present invention relates to an alignment apparatus for a movable member and, more particularly, to an alignment apparatus for a movable member, which requires high-precision alignment.
FIG. 4 shows an example of an X-Y stage of an alignment apparatus. Referring to FIG. 4, reference numeral 70 denotes a base; 71 denotes an X-stage; 72, a Y-stage; 73 denotes a yaw guide; 74 denotes an air pad; 75 denotes a linear motor for driving the X-stage; 76 denotes a linear motor for driving the Y-stage; 77 denotes a measurement mirror; 78 denotes a Y-stage position measurement mirror surface; 79 denotes an X-stage position measurement mirror surface; 80 denotes an X-stage position measurement laser beam emitted by a laser interferometer; and 81 denotes a Y-stage position measurement laser beam emitted by a laser interferometer. The X-stage 71 and the Y-stage 72 are respectively direct drive stages driven by a linear motor using a static pressure air pad. The X-stage position measurement laser beam 80 emitted by the laser interferometer irradiates the X-stage position measurement mirror surface 79 to measure the position of the X-stage 71, and the X-stage 71 is aligned to a target position. The Y-stage position measurement laser beam 81 emitted by the laser interferometer irradiates the Y-stage position measurement mirror surface 78 to measure the position of the Y-stage 72, and the Y-stage 72 is aligned to a target position.
FIG. 5 is a block diagram of an alignment apparatus for controlling the X-Y stage having the X-stage 71 and the Y-stage 72 shown in FIG. 4. Referring to FIG. 5, reference numeral 1 denotes an X-Y stage having the X-stage 71 and the Y-stage 72; 2 denotes a motor for driving the X-Y stage 1; 3 denotes a driver for supplying a current to the motor 2; 4 denotes a compensator used for stably controlling the X-Y stage 1 with high precision; 5 denotes a subtracter for calculating the difference between the current position and the target position; 6 denotes a laser interferometer for measuring the position of the X-Y stage 1; and 7 denotes a register for holding the target position. Note that the transfer function of the compensator 4 can be expressed by equation (1) below in the case of, e.g., PID control: ##EQU1## where s is a Laplace's operator.
Conventionally, in such an alignment apparatus, the gains, i.e., k.sub.p, k.sub.i, and k.sub.d, of the compensator 4 are fixed independently of the current position of the X-Y stage 1.
However, the mechanical characteristics of the X-Y stage 1 inevitably change, depending on the current position of the X-Y stage.
For this reason, in the above-mentioned example, since the gains of the compensator 4 are fixed independently of the current position of the X-Y stage 1, alignment precision at a certain position may be good, but the system may become unstable and oscillate at another position. If the gains are lowered to stabilize the system at every position, the alignment precision deteriorates.
The present invention has been made in consideration of the conventional problems and has as its object to provide an alignment apparatus which is stable and assures high precision at every position on the X-Y stage.